An underwater pipeline is normally assembled on the laying vessel, is laid, as it is assembled, directly off the vessel, and comprises a number of pipes joined to cover distances of hundreds of kilometers. In view of the length of underwater pipelines and cables in this type of application, they are referred to as “continuous”.
With specific reference to underwater pipelines, the pipes are of standard length, normally 12 meters, range in diameter from 0.2 to 1.5 meters, and each comprise a steel cylinder; a polymer coating to protect the steel cylinder from corrosion and heat loss; and possibly a concrete or Gunite coating to weigh the pipe down.
The pipes are joined both at on-land installations to form multiple-unit-length pipes, and on laying vessels, on which the pipes, of unit or multiple-unit length, are joined to pipes already joined to others to form the pipeline, which is laid on the bed of the body of water from the laying vessel.
Underwater pipelines are assembled and laid from laying vessels in two ways, each with its own merits, depending on the depth of the bed.
A first method is to form the pipeline on a vertical assembly line, and lay it in a substantially vertical position, so the portion of the pipeline between the laying vessel and the bed assumes a J shape. This method is particularly suitable for laying pipelines on very deep beds.
A second method is to form the pipeline on a substantially horizontal assembly line, and lay it using a laying device which, in the work position, serves to guide and support the pipeline along a curved path having a first portion above water, and a second portion below water. Pipelines laid using this method assume an S shape between the laying vessel and bed.
Regardless of which method is employed, relative movement between the pipeline, or continuous, elongated member in general, and the laying vessel must always be controlled by the laying device.
The first method jogs the continuous, elongated member in a laying direction, and comprises the steps of: clamping the continuous, elongated member by means of a first clamp assembly comprising jaws and fixed to the laying tower; clamping the continuous, elongated member by means of a second clamp assembly comprising jaws and movable with respect to the laying tower; releasing the first clamp assembly from the continuous, elongated member; moving the second clamp assembly in the laying direction together with the continuous, elongated member; clamping the first clamp assembly; releasing the second clamp assembly; moving the second clamp assembly in the opposite direction to the laying direction; and repeating the above steps from the beginning to jog down one more step.
This type of laying device is substantially described in Patent Applications GB 2,364,758; GB 2,370,335; WO 2006/02719; and WO 2007/015642.
The degree to which the continuous, elongated member is clamped firmly and prevented from moving with respect to the laws substantially depends on the amount of friction between the jaws and the continuous, elongated member, how strongly the clamp assemblies grip the continuous, elongated member, and the total contact area between the jaws and the continuous, elongated member.
Only so much pressure, however, can be exerted on the outer surface of the continuous, elongated member, over and above which, the jaws could damage the continuous, elongated member at the clamping point.
Working with deep beds and exceptionally heavy continuous, elongated members per unit length poses a critical operating condition, in which the continuous, elongated member suspended between the bed and the laying device calls for considerable total clamping force.
In many cases, operating conditions therefore call for increasing the size of the jaws, but this also has its physical limits.
To overcome this drawback, friction bearings have been proposed, made of polymer material, in which aluminium bosses are embedded to improve grip. One example of this is disclosed in U.S. Pat. No. 3,754,474, in which grip is improved by bosses embedded in the polymer material.
This solution, however, is also not without drawbacks, such as rapid wear of the friction bearings, caused by inclusion of the metal bosses, and, in some cases, the real danger of damaging the continuous, elongated member.
Document WO 2007/015642 A2 discloses a clamping system comprising a first clamping assembly having a movable clamping unit and a fixed clamping unit of the wedge-shaped self-locking type. Each clamping unit comprises an annular clamping block with a conical seat; clamping jaws with conical outer faces; and rubber layers interposed between the jaws and the conical seat. The rubber layer can slide with respect to the conical seat and undergo compression and shear stresses.
This type of clamping system does not provide any particular advantage in increasing the grip with the continuous, elongated member and has the drawback that the jaws are suspended by rubber layers heavily stressed.
To overcome these drawbacks, clamp assemblies are known, as described for example in GB 2,364,758, which comprise at least two clamping units arranged in series and simultaneously gripping two separate portions of the continuous, elongated member, thus doubling the total contact area between the clamp assembly and the continuous, elongated member, while still remaining within jaw size and maximum pressure recommendations.
This solution, however, is also not without its drawbacks, owing to the load exerted by the continuous, elongated member on the clamp assembly not being evenly distributed between the two clamping units. In fact, the first clamping unit in the laying direction absorbs most of the load exerted by the continuous, elongated member on the clamp assembly, which means even clamp assemblies with a number of clamping units fail to completely eliminate slippage and so ensure firm grip.
Document WO 01/35011 discloses a clamping system of the above-identified type wherein each jaw is connected to a piston by a wedge mechanism with the interposition of a rubber layer. This solution achieves a more uniform distribution of the forces between the clamping units. However, this solution does not avoid the slippage of the continuous, elongated member, has the drawback of exerting an excessive force on the continuous, elongated member up to damage the continuous, elongated member, and the rubber layer undergoes shear stresses.